1) Field of the Invention
The present invention relates to a centrifugal compressor that pressurizes fluid to change the fluid to compressed fluid, and in particular to an impeller for pressurizing fluid and a manufacturing method for the impeller.
2) Description of the Related Art
FIG. 20 is a sectional view of an impeller in a conventional centrifugal compressor, FIG. 21 is a sectional view along line XXI-XXI in FIG. 20, FIG. 22 is a schematic diagram of shapes in respective positions in a blade of a conventional impeller, and FIG. 23 is a graph of a flow rate per a unit area with respect to a relative inlet velocity of fluid in the conventional centrifugal compressor.
A general centrifugal compressor is constituted in which an impeller having plural blades is supported to rotate freely in a casing, an inlet passage along an axial direction with respect to this impeller is formed, and a diffuser along a radial direction is formed. Therefore, when the impeller is rotated by a not-shown motor, fluid is drawn into the casing through the inlet passage, pressurized in a course of flowing to pass the impeller, and then discharged to the diffuser, in which a dynamic pressure of the compressed fluid is converted into a static pressure.
In such a centrifugal compressor, as shown in FIGS. 20 and 21, an impeller 001 includes a hub 003 fixed to a rotary shaft 002 and plural blades 004 fixed in a radial shape in an outer periphery of this hub 003. Usually, when the blades 004 of this impeller 001 is designed, a method of determining an outer peripheral side shape (a blade shape on a shroud side) and an inner peripheral side shape (a blade shape on a hub side) in the blades 004 and determining a shape of the entire blades by connecting both the shapes with a straight line is adopted.
When the centrifugal compressor described above is applied as a centrifugal compressor having a high pressure ratio, a velocity of flow of fluid sucked by the impeller 001 exceeds a sound velocity. For example, as shown in FIG. 20, the velocity of flow is Mach number Ma≅0.7 on a hub side (H), Mach number Ma≅1.0 in the middle (M), and Mach number Ma≅1.3 on a shroud side (S). Therefore, a transonic impeller having a subsonic velocity on the hub side and a supersonic velocity on the shroud side is constituted, and a shock wave is generated, in particular, from the center to the shroud side. When this shock wave is large, there is a problem in that the flow separates and the impeller stalls, whereby efficiency and performance fall.
Thus, as a technology for solving such a problem, for example, there is a patent document 1 (Japanese Patent Application Laid-Open No. H08-049696) indicated below. In the technology described in this patent document 1, a meridional plane shape of an impeller blade is set to a shape in which a corner on an outer peripheral side of an end of a leading edge is cut diagonally with respect to the leading edge such that a magnitude of a velocity component, which flows into a blade vertically, of an airflow is smaller than a critical velocity of generation of a shock wave. This controls a relative inlet velocity of the airflow to be less than the velocity of generation of the shock wave and prevents the generation of the shock wave.
Incidentally, when the impeller 001 of the conventional centrifugal compressor is applied as a centrifugal compressor having a high pressure ratio, the middle (M) of the impeller 001 is set such that a throat width of the blades 004 adjacent to each other changes linearly between the shroud side (S) and the hub side (H). A bend of the blades 004 is designed such that a deflection angle on the hub side is large compared with that on the shroud side in order to obtain a same pressure increase on the shroud side and the hub side. As a result, as shown in FIG. 22, in the impeller 004, throat widths WSth, WMth, and WHth in a throat portion B are large compared with imaginary blade passage widths WS, WM, and WH in a leading edge portion A. In addition, a ratio of a change in a flow path area from the leading edge portion A to the throat portion B is large on the hub side and small on the shroud side.
Therefore, even if the meridional plane shape of the impeller blade is formed in the shape in which the corner on the outer peripheral side of the end of the leading edge is cut diagonally as in the patent document 1 described above, it is impossible to reduce a shock wave that is generated following the change in the flow path area.
In short, when the flow path area increases due to deflection of the blade, a Mach number increases in the middle M and on the shroud side S of the blade in a supersonic area in which a velocity of flow exceeds Mach number Ma≅1.0, and a Mach number decreases on the hub side H of the blade in a subsonic area in which a velocity of flow is smaller than Mach number Ma≅1.0. Since the flow path area is related to a flow rate per a unit area, a relation between the Mach number and the flow rate is a parabolic relation as shown in a graph in FIG. 23.
Therefore, as shown in FIG. 23, when fluid is sucked, since the flow path area increases when the fluid flows from the leading edge portion A (●) to the throat portion B (Δ), a flow rate per unit area Q at that point decreases by an amount of change on the hub side (H) ΔQH, Then, the Mach number Ma decreases from MaHA to MaHB on the hub side (H). Whereas a flow rate per unit area Q decreases by an amount of change in the middle (M) ΔQM, and by an amount of change on the shroud side (S) ΔQS, the Mach number Ma increases from MaMA to MaMB in the middle (M) and from MaSA to MaSB on the shroud side (S). In this case, as an amount of change of flow rate per unit area ΔQM is larger than ΔQS, it is understood that an amount of increase in Mach number in the middle ΔMaM is larger than an amount of increase in Mach number on the shroud side ΔMaS.
In this way, when fluid flows from the leading edge portion A to the throat portion B in the centrifugal compressor having a high pressure ratio, since a flow rate per unit area decreases following an increase in a flow path area, a Mach number increases largely, in particular, in the middle in a radial direction of the blade. Therefore, a large shock wave is generated in this part, efficiency and performance of the impeller fall, efficiency of the compressor itself falls, and a range of a flow rate, in which the compressor can operate stably, decreases.